The present invention relates to a roller structure in a printing press, in which the peripheral surface speeds (tangential velocity) of rotary members that are in contact with each other are set equal or are slightly changed.
For example, in a lithographic printing press, an image is formed on the surface of a plate mounted on the outer surface of a plate cylinder with ink and dampening water supplied by an ink unit and dampening unit. This image is transferred to paper passing between cylinders directly or through a blanket cylinder, thereby performing printing. In this printing operation, a dried ink film may be mixed in the ink to stick to the plate surface, or paper dust produced while cutting the paper may be scattered to attach to the plate surface. If printing is continued in this state, a printing trouble called hickey occurs in which some blank spots are undesirably formed in the printed portions.
In order to prevent hickey, Japanese Patent Laid-Open No. 2-187336 (reference 1) discloses an apparatus in which the peripheral speed of the plate cylinder and that of the dampening form roller are made different so the dampening form roller slips with respect to the plate cylinder, thereby removing hickey on the plate surface. In the apparatus disclosed in reference 1, a driving gear connected to the plate cylinder through a gear and a driven gear connected to the dampening form roller through a gear are loosely, rotatably mounted at a distance from each other on a gear shaft rotatably driven by a motor. A differential gear, which changes rotation of the driving gear and transmits the changed rotation to the driven gear so the transmission gear ratio changes in accordance with the rotational speed of the gear speed, is mounted on the gear shaft and interposed between the driving and driven gears.
In this conventional apparatus, the motor is stopped during printing operation, and the peripheral speed of the dampening form roller and that of the plate cylinder are set at the same value. When dust attaches to the plate surface, upon rotation of the motor, the peripheral speed of the dampening form roller is changed by the operation of the differential gear to become a speed different from the peripheral speed of the plate cylinder. The outer surfaces of the dampening form roller and plate cylinder slip against each other, thereby removing the dust.
Japanese Utility Model Laid-Open No. 2-19531 (reference 2) discloses another apparatus. The apparatus disclosed in reference 2 has a motor for driving a dampening form roller, two rotational speed detection means for detecting the rotational speeds of the dampening form roller and plate cylinder, and a controller for calculating the rotational speeds of the dampening form roller and plate cylinder on the basis of detection signals from the two rotational speed detection means. In this arrangement, when dust attaches to the surface of the plate, the controller is operated by button operation, so the peripheral speed of the dampening form roller is controlled to a speed different from the peripheral speed of the plate cylinder, so the dust attaching to the surface of the plate is removed.
In each of the above conventional dampening units, an exclusive motor is required for changing the peripheral speed of the dampening form roller to be different from that of the plate cylinder. The entire apparatus accordingly increases in size and the manufacturing cost also increases. The operation of changing the peripheral speed of the dampening form roller to a value different from that of the plate cylinder or returning it to the original value is performed during printing. Hence, the amount of water to be supplied to the plate cylinder changes, and the balance of the thickness of the water film is lost, leading to a defective printing. In the dampening unit disclosed in reference 2, since two rotational speed detection means are required for detecting the rotational speeds of the dampening form roller and plate cylinder, the manufacturing cost increases.
It is an object of the present invention to provide a roller structure in a printing press, in which downsizing is achieved.
It is another object of the present invention to provide a roller structure in a printing press, in which the manufacturing cost is reduced.
In order to achieve the above objects, according to the present invention, there is provided a roller structure in a printing press, comprising a roller supported rotatably and adapted to come into contact with two rotary members, having different peripheral surface speeds, at outer surfaces thereof, a transmission path for transmitting rotation of one of the rotary members to the roller, and driving source switching means for connecting/disconnecting the transmission path, wherein when the transmission path is disconnected by the driving source switching means, the roller is rotatably driven by the other one of the rotary members.